Selectivity control circuits



' May 27, 1941.

K. R. STURLEY 2,243,401

SELECTI VITY CONTROL CIRCUITS Filed Feb. 15, 1939 Sheets-Sheet 2ATTENUAT/ON ATTENUAT/ON Vi) fi) TOAVC BIAS SOURCE INVENTOR. KENNETH R.STURLEY A TTORNEY.

Patented May 27, 1941 @FFECE 2,243,401 SELECTEVKTY GONTRGL CHR CUITSKenneth Reginald Sturie assignor to Radio Cor corporation of Delawa-r1929, Serial No. 256,447 n April 14, 1938 Application February 15 InGreat Britai 11 Claims.

This invention relates to thermionic valve amplifiers, and moreparticularly to frequency selective thermionic valve amplifiers in whichimproved selectivity is obtained by a frequency selective feedbackcircuit, or circuits, between the output electrode and one or morecontrol, or grid, electrodes of the valve.

The invention, though not limited to its ap plication thereto, isparticularly well suited to, and primarily intended for incorporationin, the intermediate frequency amplifiers of superheterodyne receivers.By means of the invention an intermediate frequency amplifier ofbandpass characteristics, with sharply defined cut-off frequencies and asubstantially uniform response over the pass range, can be obtained; thecharacteristic having steep sides and a substantially fiat top withoutthe usual pronounced peaks common to most known bandpass amplifiers.Further, the gain obtainable is rather better than is obtainable withknown bandpass amplifiers of comparable selectivity and generallycomparable design.

According to this invention a frequency selective thermionic valveamplifier comprises a valve having at least a control grid and a screengrid, in addition to a cathode and an anode, and is characterized inthat at least one frequency selective feedback circuit is includedbetween the anode and the screen grid of the said valve.

In the drawings Fig. 1 shows one embodiment of the invention,

Fig. 2 illustrates a modification,

Fig. 3a. graphically shows a characteristic secured with low screenvoltage,

Fig. 3b illustrates the characteristic with high screen voltage, V

Fig. 4 shows still another modification.

Referring first to Fig. l, which shows one em bodiment of the inventionas applied to an intermediate frequency bandpass amplifier, the valveemployed is a pentode i. The signals to be amplified are applied at 2and through a tuned intermediate frequency transformer 3, whosesecondary coil is tuned by a condenser 4, between the control grid 5 andground; the cathode 6 being connected to ground through the usualcapacity shunted self-bias resistance combination 1, 8. The suppressorgrid 24 may be connected directly to the cathode 6, or, as indicated,may be connected thereto through a source 9 (which may be adjustable) ofnegative bias potential. The anode I0 is connected at l i to thepositive terminal of a source (not shown) of anode potential through theusual intermediate frequency 1 y, Chelmsford, England, poration ofAmerica, a 8

parallel tuned circuit I2, l3, and the usual decoupling resistance I 4in series; said tuned circuit being inductively coupled to a secondinter mediate frequency tuned circuit I5, I 6, (which may, if desired,be shunted by a damping resistance H) which feeds into the next stage(not shown).

A condenser i8 is connected between ground and the junction point of thedecoupling resistance to the tuned circuit [2, l3. The two tunedcircuits I 2-! 3 and l5-lt, are employed as in the usual way to obtain abandpass effect. In order to improve the selectivity and obtain abandpass characteristic with a flatter top and deeper, steeper sides, afeedback circuit comprising two further tuned circuits l9 and 20 is, inaccordance with this invention, connected to the screen grid 2|. grid 2|is connected to a suitablepositive potential source, preferably thepositive terminal H of the anode potential source, through the two tunedcircuits {9 and 20 in series with one another and with a. decouplingresistance 22, the said resistance being connected at one end to theterminal H and at the other end to the terminal of the tuned circuitpath, l9-20 remote from the screen grid 2!. A condenser 23 is connectedbetween ground and the junction point of the decoupling resistance 22with the tuned circuit 29. For the case of a broadcast receiver forpresent day broadcast practice the circuits I9 and 20, will preferablybe chosen to resonate one a 9 kc. above and the other 9 kc. below, theintermediate frequency.

The main difference between the circuit shown in Fig. 2 and the circuitof Fig. 1 is that in Fig. 2 the inductances in the two tuned circuits l9and 23 are coupled each to a further coil 25 or 26, and the two saidfurther coils are included in series with one another in the cathode legcircuit. These further coils are connected in such manner as to opposeregeneration which might lead to oscillation between the screen grid andcathode circuits. In this circuit, as will be appreciated, feedback inthe screen grid circuit, and, also, cathode feedback are obtained. Inpractice, the turns ratio of each coil 25, or 26, to the coil to whichit is coupled may be of the order of 1:10. As in Fig. 1 the tunedcircuits l9 and 29 are tuned on either side of the midband frequency tobe passed, and serve in effect to steepen the sides of thecharacteristic, deepen the cut-off valleys and flatten the top. The twotuned circuits [9, 2i) in the embodiment of Fig. 1 may be replaced by adouble tuned trans- More specifically, the screen i former, the primaryand secondary of which are tuned to the mid-band frequency, the primarybeing inserted in the screen grid circuit and the secondary being free.The position of the valleys may be controlled by variation of mutualinductance between the primary and secondary.

In either Fig. 1 or Fig. 2 control of the feedback may be obtained bycontrol of the suppressor grid bias potential, but a certain amount ofcontrol is also obtainable by varying the control grid bias. If desired,control of both these biases may be provided. Further, if desired, andas shown in Fig. 2, control of suppressor grid potential may be obtainedby means of a variable resistance 1 in the cathode leg circuit.

Variation of feedback over the pass region may be obtained by variationof screen grid voltage.

A change in screen grid voltage changes the frequency response over thepass region from a broad fiat curve to a peaked response. Thus Fig. 3ashows in conventional manner the peaked type of curve obtained with alow screen grid voltage, and Fig. 31) represents the flat topped type ofcurve obtainable with a high screen grid voltage. In Figs. 3a and 3b,which represent the frequency characteristic due to the feedbackcircuits only, f represents the mid-frequency of the band to be passed(the I. F. frequency), and the cut-off points are marked +9 and -9 toindicate that, in general, they will be at frequencies of (Io-+9) kc.and (f0-9) kc. Variable selectivity may thus be obtained by varying thescreen grid voltage. This may be effected manually, e. g. by means of apotentiometer connected between the positive terminal H and ground, thecenter point being taken to the screen grid through the tuned circuits.

It may, however, as shown in Fig. 4, be obtained automatically byconnecting the screen decoupling resistance 22 to the anode 21 ofanother valve 28, the grid bias for which is derived from a source (notshown) of automatic volume control voltage which is applied over lead29.

What is claimed is:'

1. In a wave transmission system, a tube of the screen grid type havingwave input and output circuits tuned to a common wave frequency,degenerative feedback means operatively associated with'the screen gridof said tube and comprising at least two resonant circuits coupled tosaid screen grid, said two resonant circuits beingoppositely mistuned byequal frequency values with respect to said common wave frequency.

2. In a wave transmission system, a tube of the screen grid type havingWave input and output circuits tuned to a common wave frequency,idegenerative feedback means operatively associated with the screen gridof said tube and comprising at least two resonant circuits coupled tosaid screen grid, said two resonant circuits being oppositely mistunedby equal frequency values with respect to said common wave frequency,and means for varying the direct current potential of said screen gridto adjust the magnitude of feedback voltage and thereby to regulate theshape of the resonance curve of the said tube circuits. 7

3. In a wave transmission system, a tube of the screen grid type havingwave input and output circuits tuned to a common wave frequency,degenerative feedback means operatively associated with the screen gridof said tube and comprising at least two resonant circuits coupled tosaid screen grid, said two resonant circuits being oppositely mistunedby equal frequency values with respect to said common wave frequency,and means for controlling the gain of said tube thereby to control themagnitude of feedback voltage.

4. In a wave transmission system, a tube of the screen grid type havingwave input and output circuits tuned to a common wave frequency,degenerative feedback means operatively associated with the screen gridof said tube and comprising at least two resonant circuits coupled tosaid screen grid, said two resonant circuits being oppositely mistunedby equal frequency values with respect to said common wave frequency,and means responsive to wave amplitude for automatically regulating thepotential of said screen grid.

5. In combination, in a bandpass amplifier network of the typecomprising a tube having at least a cathode, signal grid, screen gridand anode, a signal input circuit connected to said signal grid andcathode, a signal output circuit connected to the anode and cathode, atleast two resonant circuits included in circuit with said screen grid,said two resonant circuits being oppositely mistuned by equal frequencyvalues with respect to the operating signal frequency of said input andoutput circuits.

6. In combination, in a bandpass amplifier network of the typecomprising a tube having at least a cathode, signal grid, screen gridand anode, a signal input circuit connected to said signal grid andcathode," a signal output circuit connected tothe anode and cathode, atleast two resonant circuits included in circuit with said screen grid,said two resonant circuits being ar ranged in series with each other andbeing oppositely mistuned by equal frequency values with respect to theoperating signal frequency oi said input and output circuits.

7. In combination, in a bandpass amplifier network of the typecomprising a tube having at least a cathode, signal grid, screen gridand anode, a signal input circuit connected to said signal grid andcathode, a signal output cincuit connected to the anode and cathode, atleast two resonant circuits included in circuit with said screen grid,said two resonant circuits being oppositely mistuned by equal frequencyvalues with respect to the operating signal frequency of. said input andoutput circuits, and means responsive to signal amplitude variation foradjusting the gain of said tube. V

8. In combination, in a bandpass amplifier network of the typecomprising a tube having at least a cathode, signal grid, screen gridand anode, a signal input circuit connected to said signal grid andcathode, a signal output circuit connected to the anode and cathode, atleast two resonant cir'cuits included in circuit with said screen grid,said two resonant circuits being op positely mistuned with respect tothe operating signal frequency of said input and output circuits, andmeans reactively coupling each of said two resonant circuits to saidsignal input circuit.

9. In an intermediate frequency amplifier net-. work of the typeemploying a screen grid tube provided with intermediate frequency inputand output circuits; the improvement which comprises at least tworesonant circuits electrically connected in circuit with the screen gridof said tube, said two resonant circuits being oppositely mistuned by acommon frequency value with respect to said intermediate frequencywhereby the resonance curve characteristic of said network has steepsides and a substantially flat top.

resonance curve characteristic of said network 10 has steep sides and asubstantially flat top, and means for varying the positive potential ofthe screen grid thereby to adjust the shape of said resonance curve.

11. In an intermediate frequency amplifier network of the type employinga screen grid tube :provided with intermediate frequency input andoutput circuits; the improvement which comprises at least two resonantcircuits electrically connected in circuit with the screen grid of saidtube, said two resonant circuits being oppositely mistuned by a commonfrequency value with respect to said intermediate frequency whereby theresonance curve characteristic of said network has steep sides and asubstantially fiat top, and means reactively coupling each of saidresonant circuits to said'input circuit.

KENNETH REGINALD STURLEY.

